Re: [Qemu-devel] [PATCH 1/7] virtio-9p: handle handle_9p_output() error

[Greg Kurz]

On Wed, 21 Sep 2016 16:16:59 +0200
Cornelia Huck <address@hidden> wrote:

> On Wed, 21 Sep 2016 15:14:00 +0200
> Greg Kurz <address@hidden> wrote:
> 
> > A broken guest may send a request with only non-empty out buffers
> > or only non-empty in buffers, virtqueue_pop() will then return a
> > VirtQueueElement with out_num == 0 or in_num == 0 respectively.
> > 
> > All 9P requests are expected to start with the following 7-byte header:
> > 
> >             uint32_t size_le;
> >             uint8_t id;
> >             uint16_t tag_le;
> > 
> > If iov_to_buf() fails to return these 7 bytes, then something is wrong in
> > the guest.
> > 
> > In both cases, it is wrong to crash QEMU, since the root cause lies in the
> > guest. Let's switch the device to the broken state instead.
> > 
> > Signed-off-by: Greg Kurz <address@hidden>
> > ---
> >  hw/9pfs/virtio-9p-device.c |   14 ++++++++++++--
> >  1 file changed, 12 insertions(+), 2 deletions(-)
> > 
> > diff --git a/hw/9pfs/virtio-9p-device.c b/hw/9pfs/virtio-9p-device.c
> > index 009b43f6d045..0f09bef13392 100644
> > --- a/hw/9pfs/virtio-9p-device.c
> > +++ b/hw/9pfs/virtio-9p-device.c
> > @@ -56,13 +56,23 @@ static void handle_9p_output(VirtIODevice *vdev, 
> > VirtQueue *vq)
> >              break;
> >          }
> > 
> > -        BUG_ON(elem->out_num == 0 || elem->in_num == 0);
> > +        if (elem->out_num == 0 || elem->in_num == 0) {
> > +            virtio_error(vdev,
> > +                         "The guest sent a VirtFS request without 
> > headers");
> > +            pdu_free(pdu);
> > +            return;  
> 
> Make that 'break;' to be more consistent with the code right above?
> 

The code right above isn't an error path, unlike here. Maybe I should
even add an out_err: label to make it explicit.

> > +        }
> >          QEMU_BUILD_BUG_ON(sizeof out != 7);
> > 
> >          v->elems[pdu->idx] = elem;
> >          len = iov_to_buf(elem->out_sg, elem->out_num, 0,
> >                           &out, sizeof out);
> > -        BUG_ON(len != sizeof out);
> > +        if (len != sizeof out) {  
> 
> I always find 'sizeof foo' instead of 'sizeof(foo)' a bit jarring...
> 
> > +            virtio_error(vdev, "The guest sent a malformed VirtFS request: 
> > "
> > +                         "header size is %zd, should be 7", len);
> > +            pdu_free(pdu);
> > +            return;

And same here.

> > +        }
> > 
> >          pdu->size = le32_to_cpu(out.size_le);
> > 
> >   
> 
> Looks good to me.
>